886. The aim of the Natural System is to associate plants into groups and families according to their true natural likenesses and affinities, and thus to make an expression, so far as possible, of the Divine plan in the System of Nature.
887. It differs from the artificial arrangement: while that employs only a single character in classification, the natural system regards the total organization, and seizes upon every character wherein plants agree or disagree, and forms her associations only upon the principle of natural affinity. Hence each member of any natural group resembles the others, and a fair description of one will serve, to a greater or less extent, for all the rest.
888. The species and genera are formed on this principle of classification, as above stated, and are truly natural associations. Individuals altogether similar, cast as it were in the same mold, constitute a species. Species agreeing in nearly all respects and differing but in few constitute a genus. Thence the genera, associated by their remaining affinities into groups of few or many, by this same method are organized into Natural Orders and other departments of the vegetable kingdom.
889. Relative value of characters. Although the natural method employs every character, yet it does not regard all of equal value or importance. As a rule, the higher the physiological importance of any organ, the higher-will be the value of the characters which it affords.
890. Because, (1) the less will it be subject to variation, and (2) the more general in respect to other organs will be the resemblance of those plants which agree in respect to that organ. Thus, first in value are those characters drawn from the cellular tissue; second, from the vessels, the stomata, the embryo, and albumen; third, from the axis and leaves, the stamens, pistils, and fruit; fourth, from the perianth; fifth, from the inflorescence and bracts.
891. History of the natural method. Its foundation was first laid 1682, by John Ray, of England. Ho separated the Flowering from the Flowerless plants, and divided the former into Dicotyledons and Monocotyledons. Linnaeus, about 50 years later, constructed a fragment of the system, consisting of the names of 67 natural orders, without descriptions. But to Antoine de Jussieu is due the honor of the final establishment of this Method upon the true principles of natural affinity. He arranged the genera then known (A. D. 1780) into 100 natural orders, defining them with so much exactness, that nearly all have withstood the test of time; and have been adopted into our present systems. Robert Brown contributed largely to its completion and introduction into England, by the publication of his Flora of
New Holland in 1810; and finally De Candolle, by the publication of his great work "Prodromus Systematis Naturalis Regni Vegetabilis", commenced in 1816, and designed as the universal Flora, brought this system into general use.
892. Many systems. But the aim of the natural system as an expression of the Divine Order of Nature is not yet consummated. The lower divisions of the System - species, genera, and orders, - are well defined and settled as truly natural groups. The grand divisions also, - Cryptogamia, Phenogamous Exogens and Endogens (§ 897). - are fully established. But in the midst, between these extremes, there is a region of uncertainty, respecting the arrangement of the orders into groups subordinate to the grand divisions, viz., into Classes and Subclasses. In this matter, authors have maintained different views, and proposed a variety of systems.
893. The difficulty lies in this. Wo attempt necessarily a linear arrangement of the orders, placing one after another in succession, thus regarding the affinities of each in two directions only, viz., toward the preceding and the succeeding; whereas each order should be regarded as a, center of affinities; being related immediately to all others lying around it, as a province on a geographical map is related equally to all those which touch upon its borders. Hence any linear arrangement of the orders must be in some degree artificial.
894. One natural system. Although there be but one truly natural system, yet while any portion of it remains imperfectly understood, so far authors may be expected to hold different views, and to attempt by different methods to express that true system. Still the discrepancies observed in the several "Natural Systems," proposed by different writers, are slight compared with the number and importance of the principles now common to them all and universally admitted.
895. The first and highest division of the Vegetable Kingdom, viz., into the Phaenogamia or Flowering Plants, and the Cryptogamia or Flowerless Plants, has already been noticed and defined. These grand divisions, or sub-kingdoms, lie at the foundation of both the systems of Linnaeus and of Jussieu, and have in substance been adopted by every subsequent author. It is a division founded in nature, that is, marked by nature herself, for a, Are generally destitute of a regular axis and of true leaves, i, They possess mostly a cellular structure only, c, They do not develop true flowers, d, They produce spores having no embryo.
The Phaenogamia (фaivω, to appear, yaµος, nuptials), a, Consist of a regular axis of growth with leafy appendages, b, They possess a woody and spiro-vascular structure, c, They develop stamens and pistils constituting flowers.
d, They produce seeds containing an embryo. On the other hand The Cryptogamia (kρvπτω, to conceal, yuµoς).
896. Natural divisions indefinite. The above diagnosis does not mark the absolute limits between the two sub-kingdoms, for the higher Cryptogamia, as the mosses and ferns, give indications of approach to the Phaenogamia, both in form and structure, while the lower Phaenogamia can scarcely be said to produce flowers. And universally, so gradual is the transition from group to group, that it is difficult a, Growing by scattered internal wood-bundles, b, Leaves parallel-veined, c, Flowers very generally 3-parted, d, Seeds with one cotyledon, and e, The radicle never producing an axial root.